Twin Peaks is a tremolo that consists of two separate analog audio paths whose volume can be modulated.
One is a low-pass and the other a high-pass filter.
When you modulate both in sync it sounds like a regular tremolo where the whole audio spectrum is modulated at once.
When you modulate them out of sync your output signal will pan from the low pass to the high pass signal and you will get interesting phasing effects where the two filter curves meet.
This is what is usually referred to as a harmonic tremolo.
While the Tap tempo and LFO functions are digital, the audio path is 100% analog. The tremolo effect is achieved using optocouplers.